ctypes pointer to structure memory swap

[Wanderer]

On Mar 18, 6:42 pm, Nobody <nob... at nowhere.com> wrote:
> On Fri, 18 Mar 2011 15:16:40 -0700, Wanderer wrote:
> > Thanks for the reply, but I'm still not sure I understand. Why should
> > Object1 be at address1 and Object2 be at address2 and the next moment
> > Object2 is at address1 and Object1 is at address2? I'll try casting
> > them to see what the value is before and after calling
> > ReleaseCameraSettingStruct.
>
> QCamSettingsEx is a ctypes Struct object. The pSettingsID "field" is a
> ctypes field, not a Python field. It will contain a "bare" pointer to the
> C-struct for the QCamSettingId object.
>
> The point of a ctypes Struct is to store data in a format usable by C
> code, i.e. the memory layout will match that of a C struct. In particular,
> pointers within that struct will be pointers to C-compatible data (e.g. C
> structs), not pointers to Python objects. If you access the fields from
> Python, ctypes will generate Python objects on-the-fly from the raw data.
>
> Whenever you read the pSettingsID field, ctypes will generate a ctypes
> pointer object (LP_QCamSettingId?) which wraps the underlying C pointer.
> If you dereference that, ctypes will generate a Python QCamSettingId
> object which wraps the C struct.
>
> As you aren't storing the references, as soon as the print statement
> completes, the reference count will drop to zero and the object will be
> deallocated. The next time you reference pSettingsID or pSettingsID[0], a
> new object will be constructed. The addresses (and thus IDs) of the Python
> objects are entirely arbitrary.

Great explanation. Now I understand. Thanks